Astronomers
using NASA's Hubble Space Telescope have taken attendance in a
class of brown dwarfs and found indications that these odd and
elusive objects also tend to be loners.
The Hubble
census -- the most complete to date -- provides new and compelling
evidence that stars and planets form in different ways.
"Because
the brown dwarfs bridge the gap between stars and planets, their
properties reveal new and unique insights into how stars and planets
form," said Joan Najita of the National Optical Astronomy
Observatory in Tucson, AZ.
 Right:
The approximate size of a brown dwarf (center) compared to Jupiter
(left) and the Sun (right). Although brown dwarfs are similar
in size to Jupiter, they are much more dense and produce their
own light whereas Jupiter shines with reflected light from the
Sun. (Illustration: CXC/K.Kowal)
Considered
an astronomical oddity only a few years ago, brown dwarfs are
intriguing objects that, unlike stars, are too low in mass to
burn hydrogen, but are more massive than planets. At 15 to 80
times the mass of Jupiter, the light that they emit is so faint
it's hard to tell how many of them are scattered throughout the
galaxy, and how they're formed.
The Hubble
census finds that, like stars, there are more low-mass brown dwarfs
than high-mass ones, and this trend continues down to low, nearly
"planetary" masses. "In this respect, the isolated,
or free-floating, brown dwarfs found by Hubble appear to represent
the low-mass counterparts of the more massive stars. This suggests
that stars and free-floating brown dwarfs form in the same way,"
added Najita.
However, the
Hubble finding also offers the strongest evidence so far that
free-floating brown dwarfs are far different than the recently
discovered planets that orbit nearby stars. Najita's team found
brown dwarfs more often alone than in orbit around other stars.
"This suggests that the extra-solar planets and, by extension,
the planets in our own solar system, formed very differently from
how the Sun and other stars formed," Najita noted.
Only a few
years ago, it was commonly believed that brown dwarfs were rare,
perhaps because the star-forming process "stops working"
at lower masses. "Nature does not discriminate between stars
that can shine by fusion and lower-mass objects that are unable
to do so," said Najita. "In fact, the universe easily
makes brown dwarfs of all masses, from the most massive to the
least."
The study
also found that brown dwarfs are unlikely to contribute significantly
to the mysterious, unseen "dark matter" that dominates
the mass of our galaxy and the universe. Although Hubble found
that brown dwarfs are abundant, it turns out that they are not
common enough to explain the dark matter. Najita and her colleagues
conclude that brown dwarfs probably contribute less than 0.1 percent
of the mass of our Milky Way's halo.
Above: Hubble's near-infrared camera recently revealed about 50
newborn brown dwarfs throughout the Orion Nebula's star-forming
Trapezium cluster. Hajita, Tiede and Carr used the HST to examine
brown dwarfs in another young cluster, IC 348, to reach the conclusions
described in this story. Hubble's ability to detect faint brown
dwarfs in clusters like these is allowing researchers to make
great strides in understanding how stars and planets form. [more
information]
The inventory
was carried out using Hubble's infrared vision to measure the
brightness and temperature of stars in the cluster IC 348, located
in the constellation Perseus. Because the cluster is young, the
brown dwarfs in the cluster are intrinsically brighter, which
made it easy to detect about 30 brown dwarfs. A critical step
in the observation was picking out the brown dwarfs from the clutter
of background stars. To tackle this problem, Najita and colleagues
developed a new technique using Hubble's NICMOS camera. The procedure
measures the strength of an infrared water-absorption band in
the atmospheres of stars. The strength of the band is a sensitive
measure of each star's temperature.
"The
ability to measure the temperature of each star solved several
problems simultaneously," Najita said. "In addition
to helping us distinguish the cluster brown dwarfs from the background
stars, we were also able to measure the masses of the brown dwarfs
without having to assume their age. This greatly improved our
mass estimates."
Najita's study
with fellow National Optical Astronomy Observatory researcher
Glenn Tiede and John Carr of the Naval Research Laboratory, Washington,
DC, will appear in the October issue of the Astrophysical Journal.
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